Resin compositions prepared by reinforcing polyamide resin with inorganic fillers such as glass fiber, carbon fiber, talc and clay are widely known. However, these reinforcing materials improve the mechanical properties and the heat resistance unfortunately only when mixed in large amounts, and the obtained resin compositions are disadvantageously increased in mass because these reinforcing materials are high in specific gravity.
When glass fiber or carbon fiber, for example, is used as the reinforcing material, the molded article made of the obtained resin composition unfortunately undergoes a large warping. Also, in the case where clay or talc, for example, is used as the reinforcing material, when the obtained resin composition is discarded, these reinforcing materials remain as burned residues, and hence unfortunately are buried in soil and semipermanently remain in soil.
Recently, cellulose has been used as reinforcing materials for resin materials. Examples of cellulose include celluloses obtained from wood, celluloses obtained from non-wood resources such as rice straw, cotton, kenaf and hemp, and bacterial celluloses produced by microorganisms; cellulose is found in very large quantities on the earth. Cellulose is excellent in mechanical properties, and accordingly, the inclusion of cellulose in a resin is expected to result in an effect to improve the properties of the resin composition.
As the method for including cellulose in a thermoplastic resin, a method in which the resin and the cellulose are melt-mixed with each other is general. However, in this method, cellulose is mixed in the resin while the aggregated condition of the cellulose is being maintained, and hence a resin composition in which the cellulose is uniformly dispersed cannot be obtained. Consequently, the properties of the resin composition cannot be sufficiently improved.
For example, Patent Literature 1 discloses a composite material including a cellulose pulp fiber in a thermoplastic resin, and also describes a polyamide resin as the thermoplastic resin. In this invention, it is also described that the cellulose pulp fiber is made granular by using, for example, a rotary knife cutter, for the purpose of allowing the cellulose pulp fiber to be easily mixed with the polymer material. However, in Patent Literature 1, it is described that when the cellulose pulp fiber is made granular and the fiber length is made short, the reinforcing capability due to the addition of the cellulose pulp fiber is degraded, and hence the average length of the cellulose pulp fiber is preferably 0.1 to 6 mm.
In the invention of Patent Literature 1, the cellulose pulp fiber is mixed in the thermoplastic resin in a large amount, and in Example of Patent Literature 1, the cellulose pulp fiber is added in a large amount of 30% by mass.
Additionally, in the invention of Patent Literature 1, when the cellulose pulp fiber is mixed with the polymer material, the cellulose pulp fiber is dried and then the melt-mixing of the cellulose pulp fiber and the polymer material is performed.
As can be seen from what has been described above, in the invention of Patent Literature 1, the problem of the aggregation of the cellulose pulp fiber is not yet solved, and additionally, due to the large addition amount of the cellulose pulp fiber, unfortunately coloration ascribable to the decomposition of the cellulose also occurs at a temperature of 230 to 240° C. at the time of injection molding.
Patent Literature 2 describes a thermoplastic resin including 0.01 to 20 parts by weight of cellulose fiber in relation to 100 parts by weight of the resin. In Patent Literature 2, it is also described that the cellulose fiber is a viscose fiber, and the viscose fiber having a fiber length of 50 μm to 5 mm or a fiber diameter of 1 to 500 μm is preferable. In the invention described in Patent Literature 2, the content of the cellulose fiber is smaller than in the invention described in Patent Literature 1, but the fiber length or the fiber diameter of the cellulose fiber is larger, and as a method for including a cellulose fiber, only a melt-mixing method is presented.
Accordingly, also in the invention of Patent Literature 2, such a problem of aggregation of cellulose fiber as described above is not solved.